The present invention relates to a servo signal generator for an optical disk driving apparatus.
Japanese Patent Application Laying Open (KOKAI) No. 63-90035 discloses an optical disk driving apparatus which comprises a so-called sample-servo type optical disk and a tracking signal detection means.
The known sample-servo type optical disk has concentric circular or spiral tracks on which a data area for recording information and a servo area for positioning or seeking the track are arranged. In the servo area, a plurality of central data servo pits are formed on and along the center line of the track. In the servo area are also formed a plurality of first offset servo data pits and second offset servo data pits which are offset from the center line of the track. The first offset servo data pits are disposed on one side of the center line of the track while the second offset servo data pits are disposed on the other side of the center line of the track.
The above-mentioned tracking signal detection means comprises a quarter division optical detector which divides rays reflected from the optical disk surface to four parts and detects the reflected rays of each of the four parts and an adder circuit which adds the detection signal from each of the divided parts in a predetermined way to obtain a reading signal.
When a reading laser beam spot scans on and along the center line of the track, the levels of the reading signals from the first and second offset servo data pits are the same since the distance from the spot point to each of the first pits is the same as that from the spot point to each of the second pits. Whereas when the reading laser beam spot scans on and along a line offset from the center line, the level of the reading signal from the first offset servo data pits is not equal to that from the second offset servo data pits since the distances from the beam spot point to the first and the second offset pits are not the same together.
The reading signal from the adder circuit is transmitted to two sample hold circuits to sample the reading signals from the first and second offset servo data pits, respectively. The output signals from the two sample hold circuits are transmitted to a subtracter circuit to obtain a tracking signal in response to the difference between the output signals from the two sample hold circuits.
Also, in general, the beam spot is checked to determine whether the scanning line is aligned with the track center so that the beam spot accurately follows the track when the spot scans the track for reading or writing information from or to the track. Such a check of track misalignment is necessary to avoid misreading or miswriting of information due to misalignment of the scanning line with the track.
In accordance with a conventional track misalignment detection circuit, a window comparator comprising two comparators and an OR circuit is used to detect the above-mentioned track misalignment of the optical spot by checking whether the tracking signal is more than an upper threshold level determined by a power source or less than a lower threshold level. The OR circuit outputs a track misalignment signal when the tracking signal exceeds the upper threshold level or becomes less than the lower threshold level. The track misalignment signal is not output from the OR circuit when the tracking signal is between the upper and lower threshold levels.
However, in accordance with the conventional optical disk driving apparatus using the above-mentioned track misalignment detection circuit, the amplitude of the tracking signal changes according to the reflection factor of the optical disk so that the distance between the optical spot and the track center is detected improperly since the distance corresponds to the amplitude of the tracking signal. This causes problems that a track misalignment signal is mistakenly generated from the detection circuit although the misalignment amount is negligible or to the contrary the track misalignment signal is not generated although the optical spot is not aligned with the track center. Such problems further result in that it becomes impossible to read or write information from or to the right track or that recorded information is mistakenly erased by miswriting information to a wrong track.